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| United States Patent |
5,279,777
|
|
Kojima
, et al.
|
January 18, 1994
|
Process for the production of friction materials
Abstract
A process for producing a friction material under low-temperature
conditions using, as a reinforcement, one or more fibers selected from a
metal fiber, an organic fiber and an inorganic fiber and, as a binder, a
compounded pitch that consists of a mesophase pitch and sulfur and/or an
aromatic nitro compound. The content of an optically anisotropic phase in
the mesophase pitch is at least 80% and the softening point of the
mesophase pitch is not higher than 270.degree. C. The friction material
produced exhibits consistent friction characteristics over a broad
temperature range.
| Inventors:
|
Kojima; Takashi (Ibaraki, JP);
Sakamoto; Hitoshi (Ibaraki, JP);
Kamioka; Nobuo (Saitama, JP);
Tokumura; Hiroshi (Saitama, JP)
|
| Assignee:
|
Mitsubishi Gas Chemical Co., Inc. (Tokyo, JP);
Akebono Research and Development Centre, Ltd. (Hanyu, JP)
|
| Appl. No.:
|
967426 |
| Filed:
|
October 28, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
264/29.5; 264/29.6; 264/29.7; 264/85; 264/325; 264/345 |
| Intern'l Class: |
B29C 043/56; C01B 031/02 |
| Field of Search: |
264/29.5,29.6,29.2,29.7,345,85,325
208/44
423/449
|
References Cited
U.S. Patent Documents
| 3970174 | Jul., 1976 | Kirkhart | 264/29.
|
| 4209500 | Jun., 1980 | Chwastiak | 264/29.
|
| 4339021 | Jul., 1982 | Kosuda et al. | 264/29.
|
| 4348490 | Sep., 1982 | Ogiwara.
| |
| 4415363 | Nov., 1983 | Sanftlehen et al.
| |
| 4457967 | Jul., 1984 | Chareire et al.
| |
| 4476256 | Oct., 1984 | Hamermesh.
| |
| 4537823 | Aug., 1985 | Tsang et al. | 264/119.
|
| 4883617 | Nov., 1989 | Benn et al. | 264/29.
|
| 4923661 | May., 1990 | Russo | 264/119.
|
| Foreign Patent Documents |
| 62-273231 | Nov., 1987 | JP.
| |
| 63-219924 | Nov., 1988 | JP.
| |
| 2-044019 | Feb., 1990 | JP.
| |
| 3-237062 | Oct., 1991 | JP.
| |
Primary Examiner: Thurlow; Jeffery
Attorney, Agent or Firm: Venable, Baetjer, Howard & Civiletti
Claims
What is claimed is:
1. In a process for the production by a hot molding technique of a friction
material using as a reinforcement one or more fibers selected from the
group consisting of a metal fiber, an organic fiber and an inorganic
fiber, the improvement wherein a compounded pitch that consists of 70-99%
of mesophase pitch and 30-1% of sulfur and/or an aromatic nitro compound
is used as a binder, the content of an optically anisotropic phase in said
mesophase pitch being at least 80% and said mesophase pitch having a
softening point of no higher than 270.degree. C.
2. A process according to claim 1 wherein the aromatic nitro compound is
selected from the group consisting of nitrobenzene, dinitrobenzene,
dinitrotoluene, dinitrocresol, nitronaphthalene, dinitronaphthalene,
nitroanthracene, dinitroanthracene and mixtures thereof.
3. A process according to claim 1 wherein the aromatic nitro compound is
dinitronaphthalene.
4. A process according to claim 1 wherein the metal fiber is selected from
the group consisting of a steel fiber and a copper fiber.
5. A process according to claim 1 wherein the organic fiber is selected
from the group consisting of an aramid fiber and an aramid pulp fiber.
6. A process according to claim 1 wherein the inorganic fiber is selected
from the group consisting of a glass fiber, an Al.sub.2 O.sub.3 -SiO.sub.2
fiber, a potassium titanate fiber and rock wool.
7. A process according to claim 1 wherein the friction material further
contains a friction modifier and a filler.
8. A process according to claim 7 wherein the friction modifier and the
filler are each a metallic, inorganic or organic powder or short fiber.
9. A process according to claim 8 wherein the friction modifier and the
filler are each selected from the group consisting of a copper powder, an
iron powder, a zinc powder, BaSO.sub.4, Sb.sub.2 O.sub.3, Fe.sub.3 O.sub.4
and ZrSiO.sub.4.
10. A process according to claim 1 wherein the binder is used in an amount
of 3-40% on the basis of the weight of the friction material.
11. A process according to claim 1 wherein the friction material is
produced by hot compression molding.
12. A process according to claim 1 wherein the friction material is
produced by hot molding.
13. A process according to claim 12 wherein the molding temperature is in
the range of 180.degree.-400.degree. C.
14. A process according to claim 1 wherein the friction material as molded
is heat treated in an inert atmosphere at a temperature of 1000.degree. C.
and below.
15. A process according to claim 14 wherein the inert atmosphere is a
nitrogen atmosphere.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for the production of friction
materials. More particularly, it relates to a process for producing
high-performance friction materials that are markedly improved in friction
characteristics, wear resistance, heat resistance and anti-fade quality so
that they are particularly suitable for use in brakes and clutches.
Friction materials for use in brakes, clutches, etc. are conventionally
produced by binding a friction modifier, a filler and a reinforcement with
a thermosetting resin such as a phenolic resin, and molding the blend.
Friction materials, in particular those for use in brakes, are required to
exhibit extremely high performance in terms of friction characteristics,
wear resistance, heat resistance and anti-fade quality and the greater
part of their performance is determined by the performance of the binder
resin. With conventional friction materials which use phenolic resins and
other thermosetting resins as binders, it is impossible to completely
eliminate the problems that result from the deterioration of binders with
time and their low heat resistance, i.e. the change in friction
characteristics, wear, thermal crack and fade. Under the circumstances,
studies have been made of using other thermosetting resins as binders
(e.g. furan resins and modified phenolic resins) but, as of today, no
friction materials have yet been produced that far excel the prior art
versions in performance.
Another class of friction materials that have been the subject of extensive
studies are those which use sintered metals or carbon/carbon composites
unlike the conventional friction materials which use thermosetting resins
as binders. However, sintered metals involve serious problems such as
"vapor lock" due to their high heat conductivity and fusion of the
friction surface under high load, whereas carbon/carbon composite
materials suffer from the problem that they exhibit only instable friction
characteristics and low frictional resistance during low-and medium-speed
running due to their inherent sliding quality and that they will wear
rapidly. Further, the two kinds of friction materials have a common
disadvantage in that their production rate is so low that they are very
expensive. As a result, friction materials made of sintered metals or
carbon/carbon composite materials find very limited use.
With a view to solving these problems of the prior art, the present
inventors previously studied a friction material using mesophase pitch as
a binder and developed a friction material of very high performance that
was capable of exhibiting consistent friction performance even under
high-load conditions (Japanese Patent Public Disclosure No. 219924/1988).
However, to produce that friction material, hot molding under
high-temperature conditions of 400.degree.-650.degree. C. and, hence, an
expensive press unit are necessary, which is an obstacle to the
implementation of commercial production.
SUMMARY OF THE INVENTION
The present inventors hence conducted intensive studies in order to develop
an industrially advantageous process for the production of
high-performance friction materials. As a result, they found that when a
pitch that had sulfur and/or an aromatic nitro compound contained in
mesophase pitch having low softening point was used as a binder, molding
could be achieved under low-temperature conditions to yield a friction
material of very good performance. The present invention has been
accomplished on the basis of this finding.
The present invention provides a process for producing a friction material
that uses a binder a compounded pitch that consists of 70-99% of mesophase
pitch and 30-1% of sulfur and/or an aromatic nitro compound, the content
of an optically anisotropic phase in said mesophase pitch being at least
80% and said mesophase pitch having a softening point of no higher than
270.degree. C.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a graph showing the results of the brake dynamo test that was
conducted in Examples 1 and 2 and in Comparative Examples 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described below in detail.
The term "optically anisotropic phase" as used herein means that part of a
sample (a pitch mass that was solidified near at ambient temperature and
which was polished on a cross-sectional surface) which, when examined with
a reflecting optical microscope under crossed Nicols, produces observable
brilliance with the sample or the crossed Nicols being rotated (i.e.,
optically anisotropic). The term "the content of an optically anisotropic
phase" means the percent area of that optically anisotropic phase as
examined with a microscope. The term "mesophase pitch" means a pitch that
contains this optically anisotropic phase. The term "softening point"
means the solid-liquid transition temperature as measured on the pitch
with a Kohka type flow tester.
The aromatic nitro compound that can be used in the present invention is
exemplified by but by no means limited to nitrobenzene, dinitrobenzene,
dinitrotoluene, dinitrocresol, nitronaphthalene, dinitronaphthalene,
nitroanthracene and dinitroanthracene. Such aromatic nitro compounds may
be used either independently or as admixtures; if desired, they may
contain isomers. For reasons of commercial availability and easy handling,
dinitronaphthalene can be used with advantage.
According to the present invention, a pitch that consists of 70-99% of
mesophase pitch and 30-1% of sulfur and/or an aromatic nitro compound is
used as a binder for friction materials, the content of an anisotropic
phase, as defined above, in said mesophase pitch being at least 80% and
said mesophase pitch having a softening point of no higher than
270.degree. C. If the content of sulfur and/or an aromatic nitro compound
in the pitch to be used in the present invention is less than 1%,
satisfactory impregnation cannot be achieved at low temperature; if their
content is more than 30%, they will remain unreacted in the molding. In
either case, the resulting friction material has lower strength. This is
also true with the case of using mesophase pitch that softens at higher
than 270.degree. C.; satisfactory impregnation is not achieved at low
temperature and only poor molding will result.
In the process of the present invention, the compounded pitch described
above is used as a binder to produce a friction material. The friction
material produced contains a reinforcement such as a metal fiber, an
organic fiber or an inorganic fiber. Exemplary metal fibers include steel
fibers and copper fibers; exemplary organic fibers include aramid fibers
and aramid pulp fibers; and exemplary inorganic fibers include glass
fibers, Al.sub.2 O.sub.3 -SiO.sub.2 fibers, potassium titanate fibers and
rock wool.
The reinforcement is preferably used in combination with other additives
such as a friction modifier and a filler. The terms "friction modifier"
and "filler" designate metallic, inorganic or organic powders and short
fibers, as specifically exemplified by a copper powder, an iron powder, a
zinc powder, BaSO.sub.4, Sb.sub.2 O.sub.3, Fe.sub.3 O.sub.4, ZrSiO.sub.4
etc. Other powders and short fibers may be used in accordance with a
specific object but without any other limitations.
The amount in which the binder is to be used in the present invention is
not limited to any particular value but in the case of extreme overdose or
underdose, the strength of the friction material produced will decrease.
Hence, the binder is preferably used in an amount ranging from 3 to 40%.
The ingredients can be mixed by any method, whether wet or dry, that is
employed in the production of common friction materials.
To produce friction materials by the process of the present invention, hot
compression molding and any other hot molding methods can be employed that
are applied to the molding of common friction materials.
The compounded pitch to be used as a binder in the present invention
softens at a temperature 5.degree.-80.degree. C. lower than the softening
point of the mesophase pitch which is one ingredient of the pitch. In a
typical case, the compounded pitch exhibits satisfactory fluidity and
impregnation even at low temperatures of 200.degree. C. and below. Having
reactivity with this mesophase pitch, sulfur or aromatic nitro compound
which is the ingredient to be compounded undergoes vigorous reaction at
temperatures exceeding about 180.degree. C. and, after a heat treatment
conducted at 240.degree. C. and above, there occurs conversion to a
substantially dense carbonaceous material having high heat stability.
In addition, the carbonaceous material resulting from the compounded pitch
will experience only limited weight loss while working as a very stable
binder in a temperature range from ambient up to about 1000.degree. C.;
hence, a friction material can be molded from this carbonaceous material
even at low temperatures of about 180.degree.-400.degree. C.
The friction material produced by the process of the present invention is
molded at comparatively low temperatures and yet it experiences only
limited weight loss while exhibiting high strength in a temperature range
of from ambient up to about 1000.degree. C., with the attendant advantage
of exhibiting extremely high performance in terms of friction
characteristics, wear resistance, heat resistance and anti-fade quality.
The friction material thus molded may optionally be subjected to a heat
treatment at 1000.degree. C. and below in an inert gas atmosphere such as
a nitrogen gas.
The following examples are provided for the purpose of further illustrating
the present invention but are in no way to be taken as limiting.
COMPARATIVE EXAMPLE 1
Steel fibers (45 parts), BaSO.sub.4 (10 parts), Sb.sub.2 O.sub.3 (3 parts),
ZrSiO.sub.4 (10 parts), a graphite powder (30 parts), a Zn powder (2
parts) and a mesophase pitch (9 parts) that had a softening point of
245.degree. C. and whose content of optically anisotropic phase was 100%
were dry mixed with a mixer for 5 min. The blend was subjected to
preliminary molding, placed in a mold and hot molded at 600.degree. C. to
prepare a braking pad.
The friction characteristics of the pad were tested with a brake dynamo
meter and the result is shown by curve c in FIG. 1. Upon completion of the
test, the pad was found to have undergone a total wear of 0.35 mm. The
test result shows that the pad of Comparative Example 1 exhibited
extremely high performance in terms of friction characteristics, wear
resistance, heat resistance and anti-fade quality; however, hot molding at
600.degree. C. was necessary to produce the pad.
EXAMPLE 1
Steel fibers (45 parts), BaSO.sub.4 (10 parts), Sb.sub.2 O.sub.3 (3 parts),
ZrSiO.sub.4 (10 parts), a graphite powder (30 parts), a Zn powder (2
parts) and a compounded pitch (9 parts) that consisted of 20% sulfur and
80% of mesophase pitch having a softening point of 203.degree. C. and
whose content of optically anisotropic phase was 99% were dry mixed with a
mixer for 5 min. The blend was subjected to preliminary molding, placed in
a mold and hot molded at 250.degree. C., taken out of the mold and
subjected to a heat treatment in a nitrogen atmosphere at 600.degree. C.
to prepare a braking pad.
The pad was subjected to a dynamo test as in Comparative Example 1 and the
result is shown by curve a in FIG. 1. Upon completion of the test, the pad
was found to have undergone a total wear of 0.46 mm. The test result shows
that the pad of Example 1 was molded at low temperature but that it
exhibited as high performance as the pad of Comparative Example 1 in terms
of friction characteristics, wear resistance, heat resistance and
anti-fade quality.
EXAMPLE 2
Steel fibers (45 parts), BaSO.sub.4 (10 parts), Sb.sub.2 O.sub.3 (3 parts),
ZrSiO.sub.4 (10 parts), a graphite powder (30 parts), a Zn powder (2
parts) and a compounded pitch (9 parts) that consisted of 15% of
dinitronaphthalene and 85% of mesophase pitch having a softening point of
232.degree. C. and whose content of optically anisotropic phase was 100%
were dry mixed with a mixer for 5 min. The blend was placed in a mold, hot
molded at 300.degree. C., taken out of the mold and subjected to a heat
treatment in a nitrogen atmosphere at 600.degree. C. to prepare a braking
pad.
The pad was subjected to a dynamo test as in Comparative Example 1 and the
result is shown by curve b in FIG. 1. Upon completion of the test, the pad
was found to have undergone a total wear of 0.44 mm. The test result shows
that despite molding at low temperature, the pad of Example 2 exhibited as
high performance as the pad of Comparative Example 1 in terms of friction
characteristics, wear resistance, heat resistance and anti-fade quality.
COMPARATIVE EXAMPLE 2
Steel fibers (45 parts), BaSO.sub.4 (10 parts), Sb.sub.2 O.sub.3 (3 parts),
ZrSiO.sub.4 (10 parts), a graphite powder (30 parts), a Zn powder (2
parts) and a novolak phenolic resin (9 parts) containing 10% of hexamine
were dry mixed with a mixer for 5 min and subjected to preliminary
molding. The resulting preform was hot molded in a mold that had been
preheated to 150.degree. C. Thereafter, post-curing was conducted at
230.degree. C. to prepare a braking pad.
The pad was subjected to a dynamo test as in Comparative Example 1 and the
result is shown by curve d in FIG. 1, from which one can see that the pad
of Comparative Example 2 was low in heat resistance and faded considerably
(experienced a marked drop in friction coefficient). Upon completion of
the test, the pad was found to have undergone a total wear of 0.68 mm,
indicating that it was also poor in wear resistance.
In accordance with the process of the present invention, friction materials
that exhibit consistent friction characteristics over a broad temperature
range can be produced in an economically advantageous way under
low-temperature molding conditions.
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